In an instrument using an electron beam such as a scanning electron microscope or an electron beam metrological system, an electron beam is directed to a specimen. The produced secondary electrons or backscattered electrons are detected. An image of the specimen surface is obtained from the resulting signal. When a device on which an LSI or VLSI circuit is fabricated is observed with such an instrument, metal has not yet been deposited on the specimen. Therefore, the specimen is electrically charged by the illuminating beam. To prevent this phenomenon, an electron beam accelerated at a low accelerating voltage of 0.6 to 1.2 kV is made to hit the specimen. On the other hand, a high magnification of 50,000.times.to 100,000.times.is needed, because the width of the IC pattern to be inspected or observed is less than 1 .mu.m for LSI and VLSI circuits.
In this instrument, as the accelerating voltage decreases, the electron beam is affected more by the external magnetic field. It is known that where the accelerating voltage is varied, the effect of the magnetic field increases in inverse proportion to the square of the ratio of the accelerating voltages. For example, the effect of the magnetic field at the accelerating voltage of 25 kV is reduced by a factor of ##EQU1## as compared with the effect of the magnetic field at the accelerating voltage of 1 kV. That is, the effect of the magnetic field at the accelerating voltage of 1 kV is 5 times as large as the effect of the magnetic field at the accelerating voltage of 25 kV. Therefore, if the accelerating voltage is set to 1 kV, and if an image which is affected by the external magnetic field to the same extent as the image obtained at the accelerating voltage of 25 kV should be obtained, then it is necessary to reduce the strength of the external magnetic field by a factor of five. However, it is normally impossible to vary the external magnetic field which is an external factor. Consequently, the capability to shield the specimen against the magnetic field is required to be increased by a factor of five in order to fulfill the above requirement.
Japanese Patent Laid-Open No. 59825/1986 discloses an electron beam instrument which satisfies the above-described requirement. In particular, the housing of an electron beam exposure apparatus incorporated in the electron beam instrument is totally lined with a magnetic shield member to prevent disturbing magnetic fields from leaking into the specimen chamber. However, it follows that a sufficiently thick magnetic shield member is substantial. Hence, the cost of production of the instrument is increased greatly.
Enhancing the magnetic shielding effect by processing the steel member itself used for the production of the specimen chamber has been discussed. However, this scheme cannot be adopted for the following reason. A specimen chamber about 50 cm cube is needed to observe a wafer 8 inches in diameter from various directions, the wafer having a VLSI circuit fabricated thereon. In this case, the top surface, the bottom surface, and the four side surfaces of the specimen chamber receive a force of 1 kg per cm.sup.2 from the atmosphere. Therefore, a plate 50 cm square receives a force of about 2500 kg. For this reason, it is necessary that a material having a large mechanical strength be selected as the material of each surface in order to minimize the mechanical distortion or warp of each surface of the specimen chamber. In this way, rolled steel, forged steel, or the like is employed. Even if such steel materials are used, the plate must be as thick as about 5 cm to secure a sufficient mechanical strength between vacuum and atmosphere. That is, in the above case, the wall measures 50 cm by 50 cm by 5 cm. It may be thought that the steel material machined into this size is heated above the Curie point to enhance the magnetic shielding effect of the steel material. However, large-sized heating furnace equipment is necessitated to thermally treat this large and thick steel material. In addition, the dimensions obtained by the machining are spoiled by the thermal treatment. This may make it impossible to assemble the instrument.
FIG. 3 shows a cross section of a specimen chamber made of the above-described steel material. When an external magnetic field B is applied to this chamber 1, the magnetic flux passes through the wall of the chamber 1 having a high magnetic permeability. A circular hold H is formed around the center of the ceiling of the specimen chamber to permit insertion of a column C. Under this condition, we measured the strength of the magnetic field at various locations and have found that the field is stronger at the edges surrounded by the circles in the figure. That is, the magnetic flux passed through the wall of the chamber leaks out from the edges. We also have found that this leaking flux greatly affects the electron beam between the objective lens and the specimen.